This invention relates to a loudspeaker for audio and video applications, and more particularly, to a loudspeaker having a specially structured pole piece for improving performance of the loudspeaker including low distortion and high output power.
Loudspeakers, or speakers, are well known in the art and are commonly used in a variety of applications, such as in home theater stereo systems, car audio systems, indoor and outdoor concert halls, and the like. A loudspeaker typically includes an acoustic transducer comprised of an electromechanical device which converts an electrical signal into acoustical energy in the form of sound waves and an enclosure for directing the sound waves produced upon application of the electrical signal.
A loudspeaker comprises a coil of wire, typically referred to as a voice coil, which is suspended between a pole piece and a permanent magnet. In operation, an alternating current from an amplifier flows through the voice coil which produces a changing magnetic field around the voice coil. The changing magnetic field around the voice coil interacts with the magnetic field produced by the permanent magnet to produce reciprocal forces on the voice coil representing the current in the voice coil.
The voice coil is disposed within the loudspeaker so that it can oscillate in accordance with the reciprocal forces along the pole piece. The voice coil is attached to a cone shaped diaphragm which vibrates in response to the oscillation (reciprocal movement) of the voice coil. The vibration of the diaphragm produces acoustic energy in the air, i.e., a sound wave.
An example of structure in the conventional loudspeaker is shown in FIG. 1. The loudspeaker 11 includes a speaker cone 13 forming a diaphragm 17, a coil bobbin 25, and a dust cap 15. The diaphragm 17, the dust cap 15 and the coil bobbin 25 are attached to one another by, for example, an adhesive. Typically, the coil bobbin 25 is made of a high temperature resistant material such as glass fiber or aluminum around which an electrical winding or a voice coil 27 is attached such as by an adhesive. The voice coil 27 is connected to suitable leads (not shown) to receive an electrical input signal through the electrical terminals (not shown) noted above.
The diaphragm 17 is provided with an upper half roll 21 at its peripheral made of flexible material such as an urethane foam, butyl rubber and the like. The diaphragm 17 is connected to the speaker frame 19 at the upper half roll 21 by means of, for example, an adhesive. At about the middle of the speaker frame 19, the intersection of the diaphragm 17 and the coil bobbin 25 is connected to the speaker frame 19 through a spider (inner suspension) 23 made of a flexible material such as cotton with phenolic resin and the like. The upper half roll 21 and the spider 23 allow the flexible vertical movements of the diaphragm 17 as well as limit or damp the amplitudes (movable distance in an axial direction) of the diaphragm 17 when it is vibrated in response to the electrical input signal.
The loudspeaker 11 also comprises a magnetic assembly (magnetic circuit) formed of an air gap 41 and annular members including a pole piece 37, a permanent magnet 33, and an upper (top) plate 35. In this example, the pole piece 37 has a back plate 38 integrally formed at its bottom. The pole piece 37 has a central opening (air passage) 40 in the axial direction for dissipating heat generated by the voice coil 27.
The permanent magnet 33 is disposed between the upper plate 35 and the back plate 38 of the pole piece 37. The upper plate 35 and the pole piece 37 are constructed from a material capable of carrying magnetic flux, such as steel. Therefore, a magnetic path is created through the pole piece 37, the upper plate 35, the permanent magnet 33 and the back plate 38 through which the magnetic flux is running.
The air gap 41 is created between the pole piece 37 and the upper plate 35 in which the voice coil 27 and the coil bobbin 25 are inserted in the manner shown in FIG. 1. Thus, when the electrical input signal is applied to the voice coil 27, the current flowing in the voice coil 27 and the magnetic flux (flux density) interact with one another. This interaction produces a force on the voice coil 27 which is proportional to the product of the current and the flux density. This force activates the reciprocal movement of the voice coil 27 on the coil bobbin 25, which vibrates the diaphragm 17, thereby producing the sound waves.
In the audio sound reproduction involving such a loudspeaker, it is required that the loudspeaker is capable of producing a high output power with low distortion in the sound waves. It is known in the art that a loudspeaker is more nonlinear and generates more distortion in lower frequencies which require large displacement of the diaphragm. This invention is to improve the performance of the loudspeaker with use of a specifically designed pole piece in the magnetic assembly.
In the conventional technology, there is an example of magnetic assembly in which a cross sectional shape of the pole piece is inclined or curved to improve the speaker performance. This conventional example is disclosed in Japanese Utility Model Publication No. 6-2896, which is reproduced in FIGS. 2A and 2B. The magnetic assembly of FIGS. 2A and 2B is configured by a top plate 54, a pole piece 52 having a back plate 51, and a permanent magnet 53 sandwiched between the top plate 54 and the back plate 51. The pole piece 52 has a through hole 55 for air passage. An air gap 56 is created between the pole piece 52 and the top plate 54 for allowing reciprocal movements of the voice coil in the vertical direction of FIGS. 2A and 2B.
The inner wall of the pole piece 52 is inclined upwardly in a manner of straight line in FIG. 2A so that the inner diameter of the through hole 55 increases toward the top end. Similarly, the inner wall of the pole piece 52 is inclined upwardly in a manner of curved line in FIG. 2B so that the inner diameter of the through hole 55 increases toward the top end. According to the description in the publication No. 6-2896, such a conical shape of the upper portion of the pole piece 55 is effective in decreasing the resistance (fluid resistance of air) against the movement of the diaphragm even when the diameter of the through hole 55 is small. However, this conventional technology does not appear to function for reducing the magnetic distortion or increasing the output power of the loudspeaker.
It is, therefore, an object of the present invention to provide a loudspeaker having an improved acoustic performance which is achieved by low distortion and high output power.
It is another object of the present invention to provide a loudspeaker having an improved acoustic performance which is achieved by a unique structure of the pole piece having a curved inner surface.
The loudspeaker of the present invention has an improved acoustic performance achieved by low distortion and high output power. The loudspeaker includes a speaker frame, a diaphragm connected to the speaker frame, a voice coil which is formed on a voice coil bobbin and is connected to the diaphragm for vibrating the diaphragm, a permanent magnet having a central opening, and a pole piece disposed coaxially within the central opening of the permanent magnet to form an air gap into which the voice coil is disposed. The top area of the through hole of the pole piece is curved with an S-shape in cross section and an inner diameter of the through hole is increased toward the inner top thereof.
Preferably, an inner end of the pole piece is projected toward a direction of the diaphragm substantially higher than an upper surface of an upper plate. The S-shape of the top area of the pole piece is formed of a first curve which is projected in one transversal direction and a second curve which is projected in another transversal direction opposite to the first curve. Further, an outside surface of an upper end of the axial through hole is curved to promote smooth air flows between the inner area of the coil bobbin and the outside of the loud speaker.
According to the present invention, the loudspeaker is able to establish a force factor curve which is flat and symmetrical with respect to the axial movement of the voice coil. Thus, the magnetic assembly of the present invention provides an improved linearity in the sound reproduction. Further, the loudspeaker of the present invention is able to establish a flux density which is higher than the conventional example with respect to the axial movement of the voice coil. Thus, the loudspeaker of the present invention is able to produce a higher output power. Because of the unique structure of the pole piece, the loudspeaker of the present invention is able to achieve the improved acoustic performances noted above.